Communication networks typically make use of one of two well established transmission mechanisms; circuit switched transfer and packet switched (or just packet) transfer. Older systems tend to use the former, and in the main use time division multiplexing to divide the time domain, for a given frequency band, into time slots of equal duration. Circuits are defined by grouping together identical slot positions in successive time frames. Packet networks typically do not allocate fixed resources to transmitters, but rather route packets of data on a best efforts basis, using destination address information contained in packet headers, and network switches and routers. Packet networks are becoming more popular amongst network operators as they often provide better performance, and are more cost effective to install and maintain, than equivalent circuit switched networks.
Traditionally, telecommunication networks have made use of time division multiplexed (TDM) circuits to interconnect network switches (or exchanges). However, for the above mentioned reasons of performance and cost, many operators and leased line providers (who provide bandwidth to service providers) are moving towards replacing TDM circuits with packet networks. In many cases, switch to switch “sessions” will be provided entirely over packet networks. However, it is likely that, for many years to come, some operators will continue to rely upon TDM circuits to provide all or at least a part of the networks. This will necessitate interworking between packet networks and TDM “legacy” equipment.
FIG. 1 of the accompanying drawings illustrates schematically a carrier network 1 which is a packet switched network such as an Ethernet, ATM, or IP network. The carrier network provides leased line services to interconnect first and second customer premises 2,3, both of which make use of TDM transmitters 4,5 to handle multiple information streams. The nature of these streams is unimportant, although they could for example be voice calls, videoconference calls, or data calls. In order to facilitate the interconnection of the TDM streams, the carrier network 1 must emulate appropriate TDM circuits.
TDM links are synchronous circuits with a constant (transmission) bit rate governed by a service clock operating at some predefined frequency. In contrast, in a packet network there is no direct link between the frequency at which packets are sent from an ingress port and the frequency at which they arrive at an egress port. With reference again to FIG. 1, in order to provide TDM circuit emulation, interface nodes 6,7 at the edges of the packet network must provide interworking between the TDM links and the packet network in such a way that the TDM link at the egress side is synchronised with the TDM link at the ingress side. In other words, the TDM service frequency (fservice) at the customer premises 2 on the ingress side must be exactly reproduced at the egress of the packet network (fregen). The consequence of any long-term mismatch in these frequencies will be that the queue 10 at the egress of the packet network 1 will either fill up or empty, depending upon on whether the regenerated clock (fregen) is slower or faster than the original clock (fservice), causing loss of data and degradation of the service. Also, unless the phase of the original clock (fservice) is tracked by that of the regenerated clock (fregen), the lag in frequency tracking will result in small but nonetheless undesirable changes to the operating level of the queue 10 at the egress.
Some reliable method for synchronising both the frequency and phase of the clock at the egress of a packet network to those of the clock at the TDM transmitter must be provided. One approach is to use some algorithm to recover the transmitting clock frequency and phase from timestamps incorporated into packets by the sender, taking into account the transmission delay over the packet network. As the transmission time over the packet network is unpredictable for any given packet, an adaptive algorithm might be used. For example, some form of averaging might be employed to take into account variations in the transmission delay. For ATM, ITU standard I.363.1 and ATM Forum standard af-vtoa-0078 explain the concept of an adaptive clock recovery mechanism in general terms.
EP 1455473 discloses a technique for synchronising the clock at an egress side of a packet network to the clock of a TDM transmitter. According to this technique, the egress clock is adjusted in accordance with variations in the minimum transmit time of packets received in consecutive time periods.
The quality of the reference clock recovered at the timing destination is subject to degradation based on the other traffic on the packet network and on the packet rate used to transfer the timing information. In general, the packet rate and packet size are set as low as possible to conserve the bandwidth consumed in transporting the clock.
In general, the elements within a packet network operate by receiving a packet completely before forwarding it to the next element in the network using the appropriate port; this is known as “store and forward”. Thus, more time will be taken to forward a large packet than a small packet through such a network element. The packet delay in microseconds associated with the store and forward process for a single node of the network is shown in the following table for various packet sizes and link rates. The times specified in this table relate only to the data bits of the packets and do not take account of any post or pre-amble that may be associated with the “physical layer” of the network.
Link Speed (MHz)Pkt Size (bytes)100100010000645.1200.5120.05125620.4802.0480.20551240.9604.0960.410102481.9208.1920.8191518121.14412.1141.211
FIG. 2 of the accompanying drawings illustrates the delays experienced by small and large packets and mixed packets passing in succession through three nodes of a packet network. Because of the store and forward technique, smaller packets tend to catch up with larger packets as they all progress through the same path of a network. In order for a small packet to avoid incurring additional delays, it should not be transmitted too soon after a large packet. However, the necessary minimum interval between transmitting large and small packets is dependent on the number of elements in the network path and this cannot be predicted. Thus, the delay experienced by small timing packets is affected in a way which is disproportionate to overall network loading or traffic.